1. Field of the Invention
This invention relates to an apparatus for measuring a flow velocity of fluid by detecting a time interval of vortexes being generated at the downstream side of a member for generating vortexes (hereinafter referred to "vortex-generating member") placed in a flow of the fluid.
2. Description of the Prior Art
In flow velocity measuring apparatuses of such type as mentioned above, a thermistor has usually been employed as a means for detecting Karman's vortex street or Strouhl vortexes being produced on the downstream side of a vortex-generating member used. To the thermistor utilized as such member for detecting vortexes, constant current is always supplied in order to maintain a temperature thereof to a prescribed constant value. From the thermistor, variation in value of resistance at the time when the thermistor is cooled by means of contact with vortexes is taken out as a detection signal. Since vortexes are produced with such a time interval being in a prescribed functional relationship with respect to the flow velocity of fluid, when frequency of the detection signal taken out of the thermistor is measured, the flow velocity of the fluid can be determined. In a conventional flow velocity measuring apparatus, such detection signal is converted into DC voltage signal by a frequency-voltage converting means, and a thermistor is subjected to feedback controlling in response to the resulting DC voltage. Accordingly, response speed of detection output of the thermistor is restricted by response speed of the frequency-voltage converting circuit, i.e., time constant thereof. Furthermore, such feedback controlling requires correction of detection signal within a considerably wide area of a range of flow velocity being measurable so that the feedback circuit becomes complicated.
Temperature-resistance converting elements other than thermistor may be utilized in such flow velocity measuring apparatuses as mentioned above. However, even though any type of detection element is used, the required condition for avoiding disappearance of detection signal due to vortexes is such that a temperature of the detection element or time constant of variation in the current flowing through the detection element differs from time constant of variation in temperature on the basis of cooling by means of vortexes. However, in the case where a time constant in the feedback circuit is lengthy, the detection output is disordered at sudden change of flow velocity, and further an excess current flows temporarily through the detection element, resulting in danger of damage of the detection element due to overheating thereof.